The present invention relates to an apparatus for developing electrostatic latent image, more particularly, to a developing apparatus constituted so that developer in the apparatus is agitated and mixed with toner sufficiently, and fed to a developing roller without being spilled out from the apparatus.
FIG. 5 is a cross-sectional view of major construction of a conventional image forming apparatus.
In the image forming assembly, a photoreceptor drum 101 as image carrier unit is subjected to charge by a charging unit 102. An image of an original is exposed at an exposure position 103 by an exposure optical system to form an electrostatic latent image on a surface of the photoreceptor drum 101. The electrostatic latent image formed on the surface of the photoreceptor drum 101 is developed by a developing unit 110 to form a toner image. The toner image is transferred to a recording paper, or a transfer paper P, by a transfer electrode 104. After the image transfer, the photoreceptor drum 101 is cleaned by a cleaning unit 106. It, then, is subjected to precharging exposure by a light unit 107 to eliminate remaining charge. The above-mentioned process is repeated.
On the other hand, the transfer paper P fed through a paper feeding means 108 and a guide plate 108A and had the toner image transferred, is separated from the photoreceptor drum 101 under the effect of a separating electrode 105. The transfer paper P, then, is carried by a carrying member 109, such as belt, into a fixing unit (not shown) where the transfer paper P is subjected to fixation. After that, it is fed out.
In the electronic photocopying machine such as mentioned above, the said developing unit 110 has a toner supply unit 120 arranged thereon to supply toner T to developer D in the developing unit 110. The image developing unit 110 has a casing 111 that has an opening on the side of the photoreceptor drum 101. The casing 111 has a main stirring member 112, a developing roller 113, a first carrying member 114, and a second carrying member 115 arranged. It also has a brush height limiting plate 116 arranged in vicinity of the developing roller 113 to limit thickness of the developer D on a surface of the developing roller 113.
The casing 111 of the said image developing unit 110 has a toner hopper 122 on an upper side thereof. The toner hopper 122 has a toner supply roller 121 on a lower side thereof. Below the toner supply roller 121 of the toner hopper 122 in the down-stream carrying direction is provided an opening 117 communicated with the first carrying member 114.
With copying repeated, the toner contained in the developer D in the developing unit 110 is consumed. The toner, therefore, must be supplied as consumed to always keep a mixing ratio of the toner to carrier constant; otherwise, copying cannot be done at an optimum density. To achieve this, the developing unit 110 is arrange so that the mixture ratio of the toner to the carrier therein is detected and the toner T is automatically dropped from the hopper 122 to supply the toner as much as consumed.
With revolution of the said developer stirring member 112, the developer D is fed toward an opening A below an image developing area, and the developer D may be scattered and dropped out of the machine. This results in contamination of the photoreceptor drum 101, the image transfer electrode 104 therebelow. For a multi-color image forming machine, the dropped developer D may be mixed into other developing unit placed below, the developing unit.
In order to prevent such problems, an idea of an improved apparatus was proposed by me as in the Tokkaihei No. 1-10608 Opened Gazette. The apparatus has an additional concave developer receiver provided on a bottom of the developing unit 110 below the developing roller 113 and in vicinity of the opening A to prevent the dropping of the developer from the opening A.
However, the developer stirred by the developer stirring member 112 and spilled out the opening A also will be run over the developer receiver to drop out and scatter. These result in a problem that the inside of the apparatus is contaminated, and that a reproduced image is made dirty on its background.
FIG. 6(A) is a cross-sectional view of the apparatus. As shown in the figure, the casing 111 is formed as a channel shaped body which comprises a bottom member 111A and side wall members 111B, with its upper portion 111C made open. The open upper portion is closed by a upper lids 117 and 124.
The casing 111 has the developing roller 113 rotatably supported by the both side wall members 111B. A gap l between an inner surface of the bottom wall member 111A of the casing 111A and an outside surface of the developing roller 113, is kept equal at a certain value on an entire length thereof. If the gap l is too narrow, the developer D adhered on the outside surface of the image developing roller 113 can be clogged at the narrow gap not to be circulated by the developer stirring unit. This will cause the overflow of the developer D out of the apparatus, resulting in scattering outside to contaminate outside the apparatus. If the gap l is too wide, on the other hand, the developer D from the stirring unit of the apparatus will easily pass through the wide gap, resulting in flow out and scattering.
In general, the developing apparatus is made of resin. The casing 111, as shown in FIG. 6(B), will be curved, or deformed, when it is expanded or contracted according to the change of the outside temperature. This makes unequal the gap l between the inner surface of the bottom wall member 111A of the casing 111 and the outside surface of the image developing roller 113. More specifically, a gap l1 around the center becomes narrower than gaps l2 near the side walls 111B. This, as described above, will cause the developer to be clogged at the narrow gap around the center, resulting in flowing out. Clogging of the developer around the center, also, will brake a drive torque of the developing roller 113. This results in problems of uneven rotation of the developing roller 113 or scratches on the surface thereof.